Prevent Docker.options from being overwritten

When running beaker specs under MacOs X the following exception occurs.

Excon::Errors::SocketError: end of file reached (EOFError) (swipely/docker-api#202)

Trying to apply a fix by passing the path of cert files is over written by:

https://github.com/puppetlabs/beaker/blob/master/lib/beaker/hypervisor/docker.rb#L11

```ruby
cert_path = File.expand_path ENV['DOCKER_CERT_PATH']

Docker.options = {
  client_cert: File.join(cert_path, 'cert.pem'),
  client_key: File.join(cert_path, 'key.pem'),
  ssl_ca_file: File.join(cert_path, 'ca.pem'),
  scheme: 'https' # This is important when the URL starts with tcp://
}
```